A battery such as a lithium ion cell is used as a power source for various electronic devices. Typically, a battery (pack) is configured from a plurality of unit cells, and for the plurality of cells, charging voltages thereof become differed due to an individual difference in dynamic state caused by a Coulombic efficiency and capacity, as operation time passes.
This may cause a total discharge capability of the battery to be permanently limited, when at least one cell has a very lower charging voltage than other cells.
In addition, this may also cause a total charge amount of the battery to be permanently limited, when at least one cell has a very higher charging voltage than other cells.
When one cell has a limiting value of the lowest charging voltage and another cell has a limiting value of the highest charging voltage, the battery may not be charged or discharged, although all other cells respectively have proper charging voltages.
In addition, even in a case where charging/discharging is repeated, imbalance between cell voltages occurs and thus the battery lifetime is shortened and an energy efficiency of the battery cell decreases.
Furthermore, when cell voltage imbalance occurs in a complete product, it is difficult to provide an after-sales service to each cell, which may lead to a costly exchange of the battery pack itself.
Thus, cell balancing is necessary for the battery pack, and there are typically two cell balancing schemes of a passive scheme and an active scheme.
In the passive scheme, a voltage difference with other cells is reduced by applying a high level cell voltage across a resistor to consume the voltage as heat energy.
In the active scheme, the entire cells are made to have a uniform value by moving a high level cell voltage to a cell having a low level voltage by using an element for storing energy such as an inductor or capacitor.
However, in the passive scheme, a time required to balance is relatively short, but the balancing is performed only in a direction of lowering the voltage of the battery. As time passes, an output of the battery is continuously reduced to shorten a charging cycle and an internal temperature of the battery pack is raised as heat energy is generated through a resistor.
In addition, the active scheme has a relatively long charging cycle than the passive scheme due to low energy consumption, but a voltage of one cell is moved to another cell, which results in a long cell balancing time.
Accordingly, it is required to develop a technology having a long charging cycle in the active scheme and capable of performing rapid cell balancing in the passive scheme.